A U.S. service member died Thursday from wounds sustained in an improvised explosive device blast in northern Iraq, the U.S. military said.

The U.S. military did not release details. American advisers are with Iraqi forces in the battle for Mosul, the largest offensive yet against the Islamic State in Iraq. The serviceman was part of the Mosul operation, said a U.S. official who asked not to be named because he was not authorized to discuss the incident publicly.

On Monday, Iraqi forces launched a major offensive to retake Mosul — Iraq's second-largest city — from Islamic State control. American advisers are generally positioned with headquarters and are not engaged in direct combat.

Still, U.S. officials cautioned that the troops are still exposed to danger. "Americans are in harm's way as part of this fight," Pentagon Press Secretary Peter Cook said earlier this week.

It is the fourth U.S. combat death since U.S. troops deployed in 2014 to support Iraq's military in the fight against the Islamic State. Last year, Master Sgt. Joshua Wheeler, 39, was killed during a raid on an Islamic State prison compound.

In March, Marine Staff Sgt. Louis Cardin, 27, died when Islamic State militants attacked a firebase about 60 miles south of Mosul. In May, a Navy SEAL, Special Warfare Operator 1st Class Charles Keating IV, 31, was killed in a firefight after his quick reaction force came to aid an advisory team whose base was under attack by militants.

There are about 5,000 U.S. troops in Iraq, mostly conducting advisory and training missions. About 100 U.S. troops are embedded with Iraqi units engaged in the Mosul operation.

Iraqi Kurdish peshmerga fighters stand in an area near the town of Bashiqa, northeast of Mosul, as smoke billows on Oct. 20, 2016, during an operation against Islamic State jihadists to retake the main hub city. Kurdish forces launched a fresh push against areas held by the Islamic State group around Mosul, pressing an offensive to retake the jihadists' last major stronghold in Iraq. Safin Hamed, AFP/Getty Images

In the first week of fighting in Mosul, Iraqi security forces have advanced closer to the city limits, securing villages and towns along the way.

Iraq's prime minister said Thursday the operation to recapture the sprawling city was moving ahead faster than anticipated.

"The forces are pushing toward the town more quickly than we thought and more quickly than we had programmed in our campaign plan," Haider al-Abadi said via a video-link transmission to an international meeting in Paris.

His comments came as Kurdish peshmerga forces opened a new front in the offensive, pressing into the city from the northeast. Other Kurdish forces are coming in from the east and Iraq's army is attacking northward.

Some units are as close as 12 miles to the edge of Mosul and Iraq''s elite counterterrorism forces also entered the battle Thursday.

The majority of Mosul's 1.5 million civilians are Sunni Muslims and there are concerns the Shiite Muslim fighters may take part in reprisals against fleeing Sunni civilians. Al-Abadi vowed Thursday to protect any civilians in the city, no matter their background. "We will not allow any violations of human rights," he said.